Stripline fed notch antennae have been known as wide band array elements since the 1970's. A history of such antennae is contained in a paper entitled "Endfire Slotline Antennas" which was presented at JINA '90, Nice, France, 13-15 Nov. 1990, by Daniel H. Schaubert. Elements making up such antennae usually take the form of a planar structure with two conductors flared from a common feed point or "notch" linearly, exponentially or according to any other reasonable curve, including curves with discontinuities. These elements can be used to produce antennae with wide variations in characteristics. Generally, such antennae elements are fed at the base of the notch, which is sized to match the impedance of the transmission line thereto. The conductors spread apart to gradually increase the effective impedance until it matches the free space impedance in air. In essence, the antennae are nonresonant and act like impedance matching transformers to launch radio frequency energy from a transmission line into free space. The antennae elements are commonly constructed using photolithographic fabrication techniques on printed circuit board material. This allows their shape and size to be precisely controlled. Such antennae elements are readily combined into arrays that are useful in radio astronomy instrumentation, remote sensing, multiple beam satellite communications, and special power combining and phased arrays. At microwave frequencies, endfire slotline antennae have been used for wide bandwidth scanning arrays, and appear useful for radar and electronic warfare systems, as well as multifunction antennae apertures.
The most common method of feeding endfire slotline antennas in the microwave frequency regime, is with a microstrip or stripline. Both feed methods have advantages and disadvantages and both work on the principle of the following described microstrip to slot transition, in which quarter-wave length open circuited strip is used to reflect a short circuit to the region of the slotline, feeding a maximum of the current standing wave in the region where the slot interrupts the ground plane current. This results in maximum coupling between the lines. The quarter wave length short circuited slotline stub reflects an open circuit to the region of the stub, so that all of the coupled power travels off along the slotline of characteristic impedance and then to the antenna. Unfortunately, this take a lot of real estate and provides a relatively large area of metalization, which if in a radar environment, reflects substantial amounts of RF-radiation. Therefore, it has been desired to develop robust endfire antennas that have a minimum of reflective metal, are of minimal size, usually not much bigger than the length of the notch in height, which can be used as broad band elements in almost a limitless array with minimal radar cross-section, and whose gain can be enhanced at a predetermined area of the frequency band.